Lithium (Li) primary batteries or Li secondary batteries as one of electrochemical apparatuses are being torrentially mounted into compact portable equipment due to its property of high energy density, and growing abruptly. As a separator which is an important constituent factor of the battery, a porous film such as a polyolefin nonwoven fabric and a polyolefin-made microporous film is being use. The separator is demanded to have functions of separating a positive electrode from a negative electrode to prevent occurrence of electrical short-circuit and also allowing movement of ions present between a positive electrode and a negative electrode in an electrolytic solution.
Further, the separator is preferably thin as much as possible if it satisfies the above-described functions, so that the battery as a whole can have a large energy density.
In order to achieve the above-described functions, a porous thin film is currently used as a separator. However, the production and processing costs of the film are high and as a result, the film comes expensive. Further, the film has no capability to carry the electrolytic solution and therefore, leakage of solution from the battery to outside the parts or elution of electrode substance is readily caused, giving rise to problems in view of long-term reliability and safety of the battery.
In recent years, a so-called solid polymer electrolyte comprising a composite of a polyethylene oxide-base polymer and a Li salt is being taken notice of. For example, as the solid polymer electrolyte, Br. Polym. J., Vol. 319, page 137 (1975) describes a composite of a polyethylene oxide and an inorganic alkali metal salt, which is reported to exhibit ion conductivity. Further, it is reported that a comb-structure polymer having introduced into the side chain thereof an oligooxyethylene elevates thermal motility of the oxyethylene chain bearing ion conductivity and thereby the ion conductivity is improved. For example, J. Phys. Chem., Vol. 89, page 987 (1984) describes a polymethacrylic acid having added to the side chain thereof an oligooxyethylene and compounded with an alkali metal salt. Further, J. Am. Chem. Soc., Vol. 106, page 6,854 (1984) describes a polyphosphazene compounded with an alkali metal salt. These polymers themselves form a complex with a Li salt as an electrolyte to cause ion conduction due to the thermal motion of the polymer chain. Accordingly, holes for passing the electrolytic solution as in a currently used separator are not required. However, these polymer materials are insufficient in both film strength and ion conductivity and cannot include a large amount of electrolyte.
On the other hand, a solid polymer electrolyte impregnated with an electrolytic solution comprising a metal salt and an aprotic solvent in the continuous network of the polyethylene oxide (U.S. Pat. No. 4,792,504) is proposed. This reveals that the oxyethylene chain can be impregnated not only with a Li salt but also with an aprotic solvent, whereby a uniform ion conductor free of holes is provided. However, this polymer electrolyte can be hardly processed into a strong film and has difficulties in absorbing the electrolytic solution afterward, which is also far from such thinking. Further, due to the structure of the cross-linking agent, the polymer electrolyte has failed in achieving satisfactory ion conductivity.
Furthermore, in recent years, for use in the memory backup power source, an electrical double-layer capacitor where a carbon material having a large specific surface area, such as activated carbon and carbon black, is used as polarizable electrodes and an ion conductive solution is held therebetween, is increasing. For example, Kino Zairyo (Functional Materials), page 33 (February, 1989) describes a capacitor using a carbon-base polarizable electrode and an organic electrolytic solution, and 173rd Electrochemical Society Meeting, Atlanta, Ga., No. 18 (May, 1988) describes an electrical double-layer capacitor using an aqueous sulfuric acid solution. Further, Japanese Unexamined Patent Publication (kokai) No. 63-244570 discloses a capacitor using Rb.sub.2 Cu.sub.3 I.sub.3 Cl.sub.7 having a high electrical conductivity as an inorganic solid electrolyte.
However, electrical double-layer capacitors using a current electrolyte solution are bound to problems upon a long-term use or in reliability because the solution readily leaks outside the capacitor at an abnormal time such as use for a long period of time or application of high voltage. On the other hand, electrical double-layer capacitors using a conventional inorganic ion conductive substance have problems that the decomposition voltage of the ion conductive substance is low and the output voltage is low.
Japanese Unexamined Patent Publication (kokai) No. 4-253771 proposes to use a polyphosphazene-base polymer as an ion conductive substance for batteries or electrical double-layer capacitors. When a solid ion conductive substance mainly comprising the above-described polymer is used, there are provided advantages such that the output voltage is relatively high as compared with that obtained using an inorganic ion conductive substance, formation into various shapes is available and sealing is easy.
However, the ion conductivity of this solid polymer electrolyte is from 10.sup.-4 to 10.sup.-6 S/cm and insufficient and the takeout current is disadvantageously small. The ion conductivity may be elevated by adding a plasticizer to the solid polymer electrolyte, which, however, results in impartation of fluidity As a result thereof, the electrolyte can be difficult to be handled as a complete solid, is poor in the film strength or the film-forming property, readily causes short circuit when applied to an electrical double-layer capacitor or a battery, and generates problems in view of sealing similarly to the case of a liquid-type ion conductive substance. On the other hand, when a solid electrolyte is assembled together with a polarizable electrode into a capacitor, a problem arises such that they are difficult to be uniformly compounded into a carbon material having a large specific surface area because a solid and a solid are mixed.
The solid polymer electrolyte layer in an electrochemical apparatus such as a battery and a capacitor undertakes only the transfer of ions and when it is made thin as much as possible, the volume of the apparatus as a whole can be reduced and the energy density of the battery, and capacitor, etc. can be increased. Also, when the solid polymer electrolyte layer is made thin, the electric resistance of a battery or a capacitor can be lowered, the takeout current and the charging current can be increased, and the power density of a battery can be improved. Furthermore, corrosion of ions, particularly alkali metal ions, does not easily occur and the cycle life can be prolonged. Accordingly, a solid polymer electrolyte film improved in the film strength as much as possible, capable of reduction in the film thickness and having a high ion conductivity has been demanded.
Also, in the case where a solid polymer electrolyte is disposed between the electrodes of an electrochemical apparatus such as a battery or electrical double-layer capacitor, it is possible to maintain the distance between the electrodes by disposing a frame-like spacer between the opposing electrodes. However, it is not easy to process the electrolyte and spacer and to fabricate the apparatus. In a thin apparatus or wound type apparatus, for example, a problem is encountered in the dimensional stability in thickness of the solid polymer electrolyte layer occupying the space between the electrodes other than that occupied by the spacer, depending on the type thereof, which makes it difficult to maintain the thickness, or distance between the electrodes, precisely at a constant value. As a consequence, and/or because of its insufficient mechanical strength, the inter-electrode distance fluctuates from a place to place in the apparatus, so that short circuiting can occur readily and problems can occur in the stability of characteristics and the like of the apparatus.